Method and apparatus for controlling an electric assist steering system using an adaptive blending torque filter

ABSTRACT

A steering assist system (10) providing assist in response to a steering control signal includes a vehicle speed sensor (56) for sensing vehicle speed and for providing a speed signal having a value indicative of the sensed vehicle speed and a torque sensor (44) operatively connected to a vehicle hand wheel (12) for providing a torque signal indicative of applied steering torque. A blending filter (70, 71) is connected to the torque sensor and provides a blended filtered torque signal having a non-linear characteristic at torque frequencies less than a blending frequency and a linear characteristic at torque frequencies greater than the blending frequency. The blending filter establishing the blending frequency at a value functionally related to vehicle speed. Steering assist motor (28) provides steering assist in response to a control signal. The control signal is provided in response to the blended filtered torque signal. The blending filters filtering the torque signal so as to maintain a selectable system bandwidth during system operation.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation-in-part of U.S. patent application Ser. No.08/241,053, filed May 11, 1994, to McLaughlin et al., entitled "Methodand Apparatus for Controlling an Electric Assist Steering System Usingan Adaptive Torque Filter." This earlier filed application is herebyfully incorporated herein by reference.

TECHNICAL FIELD

The present invention is directed to an electric assist steering systemand is particularly directed to a method and apparatus for controllingan electric assist steering system to improve steering feel.

BACKGROUND OF THE INVENTION

Electric assist steering systems are well known in the art. Electricpower assist steering systems that utilize a rack and pinion gear setprovide power assist by using an electric motor to either (i) applyrotary force to a steering shaft connected to a pinion gear, or (ii)apply linear force to a steering member having the rack teeth thereon.The electric motor in such systems is typically controlled in responseto (i) a driver's applied torque to the vehicle steering wheel, and (ii)sensed vehicle speed.

In U.S. Pat. No. 3,983,953, an electric motor is coupled to the inputsteering shaft and energized in response to the torque applied to thesteering wheel by the vehicle operator. The steering system includes atorque sensor and a vehicle speed sensor. A computer receives the outputsignals provided by both the torque and speed sensors. The computercontrols the amount of steering assist provided by the motor dependentupon both the applied steering torque and the sensed vehicle speed.

U.S. Pat. No. 4,415,054, to Drutchas (now U.S. Reissue Pat. No.32,222,), assigned to TRW Inc., utilizes a D.C. electric assist motordriven through an "H-bridge" arrangement. The assist motor includes arotatable armature encircling a steering member. The steering member hasa first portion with a thread convolution thereon and a second portionwith straight cut rack teeth thereon. Rotation of the electric assistmotor armature causes linear movement of the steering member through aball-nut drivably connected to the thread convolution portion of thesteering member. A torque sensing device is coupled to the steeringcolumn for sensing driver applied torque to the steering wheel. Thetorque sensing device uses a magnetic Hall-effect sensor that sensesrelative rotation between the input and output shafts across a torsionbar. An electronic control unit ("ECU") monitors the signal from thetorque sensing device and controls the electric assist motor in responsethereto. A vehicle speed sensor provides a signal to the ECU indicativeof the vehicle speed. The ECU controls current through the electricassist motor in response to both the sensed vehicle speed and the sensedapplied steering torque. The ECU decreases steering assist as vehiclespeed increases. This is commonly referred to in the art as speedproportional steering.

U.S. Pat. No. 4,660,671, discloses an electric controlled steeringsystem that is based on the Drutchas steering gear. In the arrangementshown in the '671 patent, the D.C. motor is axially spaced from theball-nut and is operatively connected thereto through a connection tube.The electronic controls includes a plurality of diagnostic features thatmonitor the operation of the steering system. If an error in theoperation of the electric steering system is detected, the power assistsystem is disabled and steering reverts to an unassisted mode.

U.S. Pat. No. 4,794,997, to North, assigned to TRW Cam Gears Limited,discloses an electric assist steering system having an electric motoroperatively connected to the rack through a ball nut. A vehicle speedsensor and an applied steering torque sensor are operatively connectedto an ECU. The ECU controls electric current through the motor as afunction of both applied steering torque and sensed vehicle speed. Thecurrent is controlled by controlling the pulse-width-modulated ("PWM")signal applied to the motor. As the PWM increases, power assistincreases. The ECU or computer is preprogrammed with discrete controlcurves that provide steering assist values (PWM values), also referredto as torque-out values, as a function of applied steering torque, alsoreferred to as torque-in values, for a plurality of predetermineddiscrete vehicle speed values. Each vehicle speed value has anassociated torque-in vs. torque-out control curve.

U.S. Pat. No. 5,257,828, To Miller et al., discloses an electric assiststeering system having yaw rate control. This system uses a variablereluctance motor to apply steering assist to the rack member. The torquedemand signal is modified as a function of the steering rate feedback.

Known electric assist steering systems have a dynamic performancecharacteristic, known as the system bandwidth, that varies as a functionof vehicle speed. As the vehicle operator applies steering torque androtates the steering wheel back-and-forth, e.g., left-to-right-to-left,the electric assist motor is energized to provide steering assistcommensurate with the steering inputs. How the steering system respondsto a particular frequency of back-and-forth steering wheel movement isindicative of the system's dynamic performance.

The amount of local change at the electric assist motor divided by theamount of local change in steering torque applied by the driver is thesteering system gain. A time delay occurs from the time steering torqueis applied to the steering wheel to the time the assist motor responds.This time delay is a function of the frequency at which the inputcommand is applied. This is referred to as the system response time. Thesystem gain is set to a predetermined value so as to have a short systemresponse time while still maintaining overall system stability. Thesystem response time and system gain determine the system bandwidth.

The bandwidth in known steering systems varies as a function of vehiclespeed. If dynamic steering frequency or the "frequency" of a transientresponse exceeds the system bandwidth at a particular vehicle speed, thesteering feel becomes "sluggish" (felt as a "hesitation" when thesteering wheel direction is changed) since the steering assist motor cannot respond quick enough. Typically, steering system gain as well assystem bandwidth decreases as the vehicle speed increases so that systemhesitation or sluggishness becomes more noticeable as vehicle speedincreases.

SUMMARY OF THE PRESENT INVENTION

The present invention is directed to a method and apparatus forcontrolling an electric assist steering system so as to have a systembandwidth independent of vehicle speed and input torque.

In accordance with the present invention, an apparatus is provided forcontrolling a steering assist system. The steering assist systemprovides assist in response to a steering control signal. The apparatuscomprises torque sensing means operatively connected to a vehicle handwheel for providing a torque signal indicative of applied steeringtorque. Blending filter means are connected to the torque sensing meansfor providing a blended filtered torque signal having a first functionalcharacteristic at torque frequencies less than a blending frequency anda second functional characteristic at torque frequencies greater thanthe blending frequency. The apparatus further includes steering assistmeans for providing steering assist in response to a control signal, andcontrol means operatively connected to the blending filter means forproviding said control signal to the steering assist means in responseto the blended filtered torque signal. The blending filtering meansfilters the torque signal so as to maintain a selectable systembandwidth during system operation.

In accordance another aspect of the present invention, a method isprovided for controlling a steering assist system that provides steeringassist in response to a steering control signal. The method comprisesthe steps of measuring applied steering torque and providing a torquesignal indicative of the measured applied steering torque, filtering thetorque signal so as to have a first functional characteristic at torquefrequencies less than a blending frequency and a second functionalcharacteristic at torque frequencies greater than said blendingfrequency so as to maintain a selectable system bandwidth during systemoperation, providing steering assist in response to a steering controlsignal, and providing the control signal in response to the filteredtorque signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates from a reading of the following detailed description withreference to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram illustrating a power assist steeringsystem in accordance with the present invention;

FIG. 2 is a schematic drawing representation of the linearized closedloop control system in accordance with the present invention;

FIG. 3 is a graphical representation of torque-in vs. torque-out controlcurves that vary as a function of vehicle speed;

FIGS. 4A and 4B are a Bode plot of an open loop system using a fixedtorque filter;

FIGS. 5A and 5B are a Bode plot of an open loop system;

FIGS. 6A and 6B are a Bode plot of an open loop system having gainsbetween 1 and 5;

FIGS. 7A and 7B are a Bode plot of the adaptive blending filter of thepresent invention for various steering system gains; and

FIGS. 8A and 8B are a Bode plot of the blending filter of the presentinvention for various steering system gains.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, a power assist steering system 10 includes asteering wheel 12 operatively connected to a pinion gear 14.Specifically, the vehicle steering wheel 12 is connected to an inputshaft 16 and the pinion gear 14 is connected to an output shaft 18. Theinput shaft 16 is operatively coupled to the output shaft 18 through atorsion bar 20.

The torsion bar 20 twists in response to applied steering torque therebypermitting relative rotation between the input shaft 16 and the outputshaft 18. Stops, not shown, limit the amount of such relative rotationbetween the input and output shafts in a manner known in the art. Thetorsion bar 20 has a spring constant referred to herein as K_(t). Inaccordance with a preferred embodiment, the spring constant K_(t) =20in-lb/deg. The amount of relative rotation between the input shaft 16and the output shaft 18 in response to applied steering torque isfunctionally related to the spring constant of the torsion bar.

As is well known in the art, the pinion gear 14 has helical teeth whichare meshingly engaged with straight cut teeth on a rack or linearsteering member 22. The pinion gear 14 in combination with the straightcut gear teeth on the rack member 22 form a rack and pinion gear set.The rack is steerably coupled to the vehicle's steerable wheels 24, 26with steering linkage in a known manner. When the steering wheel 12 isturned, the rack and pinion gear set converts the rotary motion of thesteering wheel 12 into linear motion of the rack 22. When the rack moveslinearly, the steerable wheels 24, 26 pivot about their associatedsteering axes and the vehicle is steered.

An electric assist motor 28 is drivingly connected to the rack 22through a ball-nut drive arrangement also known in the art. Such anarrangement is fully described in U.S. Pat. No. 5,257,828, to Miller etal., assigned to TRW Inc., which is hereby fully incorporated herein byreference. When the electric motor 28 is energized, it provides powerassist steering so as to aid in the rotation of the vehicle steeringwheel 12 by the vehicle operator.

In accordance with a preferred embodiment of the present invention, theelectric assist motor 28 is a variable reluctance motor. A variablereluctance motor is desirable for use in an electric assist steeringsystem because of its small size, low friction, and its hightorque-to-inertia ratio. The motor 28, in accordance with a preferredembodiment of the present invention, includes eight stator poles and sixrotor poles. The stator poles are arranged so as to be energized inpairs designated Aa, Bb, Cc, and Dd.

The operation of a variable reluctance motor and its principle ofoperation are well known in the art. Basically, the stator poles areenergized in pairs. The rotor moves so as to minimize the reluctancebetween the stator poles and the rotor poles. Minimum reluctance occurswhen a pair of rotor poles are aligned with the energized stator poles.Once minimum reluctance is achieved, i.e., when the rotor poles alignwith the energized stator coils, those energized stator coils arede-energized and an adjacent pair of stator coils are energized.

The direction of motor rotation is controlled by the sequence in whichthe stator coils are energized. The torque produced by the motor iscontrolled by the current through the stator coils. When the motor isenergized, the rotor turns which, in turn, rotates the nut portion ofthe ball-nut drive arrangement. When the nut rotates, the balls transfera linear force to the rack. The direction of rack movement is dependentupon the direction of rotation of the motor.

A rotor position sensor 30 is operatively connected to the motor rotorand to the motor housing. The above-incorporated '828 patent shows anddescribes such a rotor position sensor 30 in detail, the description ofwhich being hereby fully incorporated herein by reference. One of thefunctions of the rotor position sensor 30 is to provide an electricalsignal indicative of the position of the rotor relative to the motorstator. For proper operation of the variable reluctance motor, includingdirection of rotation and applied torque, it is necessary to know theposition of the rotor relative to the stator.

A position sensor 40 is operatively connected across the input shaft 16and the output shaft 18 and provides an electrical signal having a valueindicative of the relative rotational position or relative angularorientation between the input shaft 16 and the output shaft 18. Theposition sensor 40 in combination with the torsion bar 20 form a torquesensor 44. The steering wheel 12 is rotated by the driver during asteering maneuver through an angle Θ_(HW). The relative angle betweenthe input shaft 16 and the output shaft 18 as a result of applied inputtorque is referred to herein as Θ_(P). Taking the spring constant K_(t)of the torsion bar 20 into account, the electrical signal from thesensor 40 is also indicative of the applied steering torque referred toherein as τ_(s).

The output of the torque sensor 44 is connected to a torque signalprocessing circuit 50. The processing circuit 50 monitors the angleΘ_(P) and, "knowing" what the spring constant K_(t) of the torsion bar20 provides an electric signal indicative of the applied steering torqueτ_(s).

The torque sensor signal is passed through a pair of blending filters.The two blending filters are constructed such that the first is a lowpass filter 70 and the second is a high pass filter 71. The filters aredesigned such that summation of the two filters is identically one forall frequencies. The low pass filter 70 allows all of the signal τ_(s)with frequency content below some blending frequency w_(b) to passthrough while rejecting all high frequency data. The high pass filterallows all of the signal τ_(s) with frequency content above someblending frequency w_(b) to pass through while rejecting all lowfrequency data. The blending filter frequency w_(b) is a function ofvehicle speed and is determined by the blending filter determinationcircuit 68. The determination of w_(b) may be accomplished using alook-up table in a microcomputer or may be accomplished using an actualcalculation in accordance with a desired control function. The low passtorque sensor output signal τ_(sL) is connected to an assist curvecircuit 54.

The assist curve circuit 54 is preferably a look-up table that providesa desired torque assist signal τ_(assist) having a value functionallyrelated to the low passed applied steering torque τ_(sL) and sensedvehicle speed. A vehicle speed sensor 56 is also operatively connectedto the assist curve circuit 54. The assist curve function may beaccomplished using a look-up table in a microcomputer or may beaccomplished using an actual calculation in accordance with a desiredcontrol function.

As is well known in the art, the amount of power assist desired for avehicle steering system decreases as vehicle speed increases. Therefore,to maintain a proper or desirable feel to steering maneuvers, it isdesirable to decrease the amount of steering power assist as the vehiclespeed increases. This is referred to in the art as speed proportionalsteering.

FIG. 3 shows preferred values of output torque τ_(assist) verses appliedinput torque τ_(sL) for various vehicle speeds. Line 60 is the torque-invs. torque-out values for what is referred to in the art as dry surfaceparking. Line 66 is the torque-in vs. torque-out values for high vehiclespeeds. Line 70 shows the direction of how values change as vehiclespeed increases. Generally, the value of the output from the assistcurve circuit 54 is referred to as τ_(assist).

Preferably, the τ_(ASSIST) value is determined according to:

    τ.sub.ASSIST =S.sub.p ×(LS)+(1-S.sub.p)×(HS)

where LS is the set of lowest speed τ_(ASSIST) values for a given lowpassed input torque, HS is the set of highest speed τ_(ASSIST) valuesfor a given low passed input torque, and S_(P) is a speed proportionalterm that varies between 1 at parking speed and 0 at a predeterminedhigh speed. This provides a smooth interpolation of values as vehiclespeed increases. This determination of the τ_(ASSIST) value is fullydescribed in co-pending patent application Ser. No. 08/212,112, toMcLaughlin et al., filed Mar. 11, 1994, and is hereby fully incorporatedherein by reference.

The high passed torque sensor signal τ_(sH) is connected to a highfrequency assist gain circuit 72. The high frequency assist gain circuit72 multiplies the high passed torque sensor signal τ_(sH) by apredetermined gain S_(c1) that is a function related to vehicle speed.The determination of S_(c1) may be accomplished using a look-up table ina microcomputer or may be accomplished using an actual calculation inaccordance with a desired control function. Modification of the highfrequency assist gain S_(c1) allows the bandwidth of the steering systemto be modified.

The outputs of the assist curve circuit 54 and the high frequency assistgain circuit 72 are summed in a summing circuit 79. The output of thesumming circuit 79 is referred to as τ_(ba) and is connected to theadaptive filter circuit 80. The two signals are combined to determinethe input τ_(ba) to the adaptive filter circuit.

The adaptive filter circuit 80 filters the input blended assist torquesignal τ_(ba). The filter is adaptive in that its poles and zeros areallowed to change as the vehicle speed changes so as to provide anoptimal control system. The combination of this filtering is referred toas adaptive blending filtering and results in a filtered torque signalτ_(m), which is referred to as the torque demand signal. The torquedemand signal is connected to a motor controller 90. The motorcontroller 90 controls energization of the motor 28 in response to thetorque demand signal τ_(m). The rotor position sensor 30 is alsoconnected to the motor controller 90. The motor controller 90 controlssteering damping in response to sensed rotor speed, as is fullydescribed in the above-incorporated '828 patent. Other inputs 94 areconnected to the motor controller 90. These other inputs 94 include anECU temperature sensor, soft-start circuitry, etc. These other inputsare also fully described in the above-incorporated '828 patent.

The output of the motor controller 90 is connected to a drive controlcircuit 96. The drive control circuit is controllably connected to aplurality of power switches 100 to control the application of electricalenergy to the electric assist motor 28. The output from the rotorposition sensor 30 is also connected to the drive control circuit 96. Asmentioned above, control of a variable reluctance motor requires thatthe relative position between the rotor and the stator be known.

Referring to FIG. 2, the linearized closed loop control system of thepresent invention is shown. The linearized closed loop control system isrequired because it is used to design the blending filter and adaptivefilter for the steering system. Rotation of the hand wheel 12 results inan angular displacement of Θ_(HW) on the steering wheel side of thetorsion bar position sensor. This angular displacement is differenced104 with the resultant angular displacement of the output shaft 18 afterit is driven in rotation by the electric assist motor by an angle Θ_(m)through the gearing ratio 110 represented by r_(m) /r_(p) where r_(m) isthe effective radius of the motor ball nut and r_(p) is the effectiveradius of the pinion. In one embodiment of the present invention, thevalues are r_(m) =0.05 in. and r_(p) =0.31 in. One radian of rotation ofthe ball nut produces r_(m) inches of travel of the rack. Similarly, oneradian of rotation of the pinion produces r_(p) inches of travel of therack. The resultant angular displacement Θ_(p) times the spring constantK_(t) gives the torque signal τ_(s). In the closed loop arrangement,switch 53 connects the output τ_(s) to the low pass/high pass filtercircuits.

The torque signal τ_(s) is passed through the low pass filter 70resulting in the low passed assist torque τ_(sL). The high passed assisttorque τ_(sH) is determined by subtracting the low frequency assisttorque from the torque signal τ_(s). The reason that τ_(sH) can bedetermined in this way is discussed below.

The continuous domain blending filters are chosen such that the sum ofthe low pass filter G_(L) (S) and the high pass filter G_(H) (S) isalways equal to one:

    G.sub.L (S)+G.sub.H (S)=1

In accordance with the preferred embodiment, the low pass filter ischosen to be a first order filter with a pole at w_(b). The high passfilter is uniquely defined by the above constraint that the sum of thetwo filters must be one. Therefore, the low and high pass filters are:##EQU1## When realizing a set of blending filters in a digital computer,those skilled in the art will appreciate that it is not necessary toconstruct separate high and low pass filter stages. Rather, the input tothe blending filters τ_(s) is passed through the low pass filterresulting in the signal τ_(sL). The high passed signal is the originalinput torque minus the low passed portion:

    τ.sub.sH =τ.sub.s -τ.sub.sL

This can be thought of equivalently as determining the low frequencyportion of the signal and simply subtracting it out of the originalsignal. The result is a signal with only high frequency information.Alternatively, one can use higher order blending filters. However, thecomplexity of the filter computations increases with filter order in adigital computer. The use of first order filters is preferred.

The low passed torsion bar torque signal τ_(sL) is connected to theassist curve circuit 54. Referring again to FIG. 2, the linearizedcontrol system includes an assist curve circuit 54 designated as a gainS_(c). The gain S_(c) is the local derivative of the assist functionwith respect to the input torque evaluated at some low passed inputtorque and speed. ##EQU2## The gain S_(c) represents how muchincremental assist τ_(ASSIST) is provided for an incremental change inlow passed input torque τ_(sL) about some nominal low passed inputtorque and vehicle speed. For example, the low speed assist curve 60 inFIG. 3 has a shallow slope as the torque is increased out of thedeadband and a steeper slope at a high input torque of 25 in-lb.Therefore, the gain S_(c) is small near the deadband and increases asthe torque increases away from the deadband. The difference in slope iseven greater for the high speed assist curve 66 of FIG. 3. For a lowpassed input torque of 10 in-lb., a large change in low passed inputtorque is required to effect even small changes in assist torque.Therefore, S_(c) is small. For an input torque of 50 in-lb., a smallchange in low passed input torque produces a large change in assisttorque. For the high speed assist curve, S_(c) is very small near thedeadband and very large at 50 in-lb. of low passed input torque.

In the linearized realization of the steering system, the low passedtorque τ_(sL) is multiplied by the local gain of the assist curve todetermine τ_(ASSIST). The low passed assist value τ_(ASSIST) is summedwith the high passed assist value. The high passed assist value isdetermined by multiplying the high passed torque sensor signal τ_(sH)times the high frequency assist gain S_(c1). The blended assist is:

    τ.sub.ba =τ.sub.ASSIST +((S.sub.c1)×(τ.sub.sH))

The pole of the blending filter w_(b) and the high frequency assist gainS_(c1) are computed as functions of speed in circuits 83 and 74respectively. The determination of w_(b) and S_(c1) may be accomplishedusing a look-up table in a microcomputer or may be accomplished usingactual calculations. The circuits 83 and 74 in the linearized closedloop transfer function of FIG. 2 form the blending filter determinationcircuit 68 of FIG. 1. The blended assist is connected to the adaptivetorque filter G_(f). The adaptive torque filter allows the vehiclesteering system to adapt to changes in the dynamics of the system thatoccur as the vehicle speed changes.

The output from the adaptive torque filter 80 is a torque demand signalτ_(m). In the closed loop arrangement, switch 55 connects τ_(m) to thesumming circuit 116. The motor provides a torque assist which is summedwith the manual assist transmitted through the pinion shaft producing atotal torque τ_(r) on the rack. This torque is applied to the transferfunction G_(m) which represents the dynamics of the steering gear. Theinput to G_(m) is the total torque applied to the motor via the rack andball nut from the input pinion and the motor and the output is the motorrotation angle. The transfer function G_(m) is referenced directly tothe motor so that the input is the total torque on the motor and theoutput is motor angle. The restoring force applied by the tires on therack is modeled as a spring force which is not shown because it isinternal to G_(m).

Three key features of the blending filter topology should beappreciated. If the local assist gain S_(c) is equal to the highfrequency assist gain S_(c1), the blended assist torque τ_(ba) isidentically equal to the measured torque τ_(s) times the gain S_(c1).This results from the fact that the sum of the low passed and highpassed filters is equal to one. If the outputs of the two filters aremultiplied by the same gain, the sum of the two outputs will just be thegain times the inputs. This characteristic of the blending filtertopology is used when designing a controller for the steering system.Also, the low frequency or DC gain of the filter stage between themeasured torsion bar torque τ_(s) and the blended assist torque τ_(ba)is set by the local gain of the assist curve S_(c). This occurs becausethe output of the high pass filter stage 71 is zero for low frequencyinputs so that all of the torque sensor signals pass through the lowpass filter. Since the assist curve is a nonlinear element providingdifferent incremental levels of assist for the same increment change ininput torque, i.e., S_(c) changes in response to input torque andvehicle speed, the DC gain of the steering system is entirely selectableand tunable by changing the assist curve. Furthermore, the highfrequency gain of the filter stage between the measured torsion bartorque τ_(s) and the blended assist torque τ_(ba) is always S_(c1). Athigh frequency, the output of the low pass stage of the blending filteris zero so that all of the torque sensor signal passes through the highpass filter stage. Since the high frequency gain of the high pass stageis S_(c1), the gain between τ_(s) and τ_(ba) is S_(c1).

The blending filters, in accordance with the present invention, have theunique characteristic of responding like a linear system to a highfrequency input signal and like a nonlinear system to a low frequencyinput signal. For example, if the steering torque signal changesrapidly, as might occur due to torque ripple from the VR electric assistmotor 28, the driver inputting a rapid input torque, or the wheelsresponding to a sudden bump in the road, the high frequency inputs arerejected by the low pass filter 70 and the response of the system wouldbe dominated by the high pass portion of the loop under these steeringconditions. However, if the input to the system is smooth and slow, thenthe high pass filter rejects the low frequency input and the systemresponse is dominated by the non-linear assist curve. The system is bothresponsive to fast inputs and can achieve any feel or assist curve forlow frequency inputs.

The filter G_(f), in accordance with a preferred embodiment, is aconstant filter that is not a function of vehicle speed. The presentinvention contemplates that this filter G_(f) would be an adaptivefilter that adapts as a function of vehicle speed. It is designed bymeasuring the open loop transfer function G_(p) as a function of speedand designing a filter that meets stability and performancespecifications for all speeds. In accordance with one embodiment of thepresent invention, the open loop transfer functions are designed to havethe same bandwidth for all speeds. The present invention is not,however, so limited, i.e., the steering system bandwidth can vary as afunction of vehicle speed.

One skilled in the art will appreciate that controller design requiresthat the system dynamics must be identified prior to designing of thecontroller. Specifically, it is necessary to identify dynamics of theopen loop transfer function. The open loop transfer function, for thepurposes of this application, occurs when the motor command τ_(m) isused as the input and the measured torque sensor signal τ_(s) is theoutput. To establish such an open loop system, switches 53 and 55 areswitched so as to remove the assist curves, blending filters, and theadaptive torque filter from the system. The transfer function ismeasured on a vehicle for a particular system using a signal analyzer tocommand the motor at various input frequencies and measuring the outputof the torque sensor with the hand wheel held in a fixed position. Thismeasured transfer function is designated as G_(p) and an example of suchis shown in FIGS. 5A and 5B. (The actual values are dependent upon theparticular vehicle application.) This measured open loop transferfunction is then used to design the adaptive torque filter 80 so thatthe steering torque loop has a desired stability and performancecharacteristics.

One skilled in the art will appreciate that the open loop transferfunction G_(p) can also be determined by creating a linear model of thedynamics of the rack, tires, motor, ball nut, electronics, etc. If G_(p)is determined from an analytical model, then all of the dynamicsinvolved in converting a torque command at the motor to a measuredtorsion bar signal must be included in the model. It is preferred tomeasure this transfer function directly as analytical models rarelymatch real world phenomenon exactly especially with regard to the phaseangle of the transfer function.

The transfer function shown in FIG. 5 was measured with the vehiclestationary on a dry, flat surface. This is commonly referred to as "drypark." The hand wheel was locked. Any controller designed using thismeasured transfer function will work well at dry park. As the vehiclespeed increases, the controller may not function as desired since theopen loop transfer function may change. The open loop transfer functionis preferably measured at several different vehicle speeds and thefilters are designed for each of these speeds. The vehicle speed ismeasured in real time and the corresponding filter is used in thecontrol determination. The control system's torque filter "adapts" tosteering dynamic changes as a function of vehicle speed. To measure theopen loop transfer function as a function of vehicle speed is difficult.Alternatively, the open loop transfer function at dry park can bemeasured and used to develop a model that correlates well to themeasured data. The model can then be used to determine the open looptransfer function at higher vehicle speeds.

Although the preferred embodiment of the present invention allows theadaptive filter to change as the dynamics of the steering system change,only the dry park condition transfer function shown in FIG. 5 is used toillustrate the design process. Once the steps required for dry park areunderstood, torque filters can be designed for different vehicle speeds.

Torque filter design is performed using classical open loop techniques.The open loop steering system transfer function G_(p) is measured and isshown in FIG. 5. The open loop transfer function that must be stabilizedincludes not only G_(p), but also any gain due to the assist curve. Froma stability point of view, the system must be stable for the case of thehighest system gain:

    S.sub.c =(S.sub.c).sub.max

Next, set both the assist curve gain 54 equal to (S_(c))_(max) and thehigh frequency assist gain 72 equal to (S_(c))_(max). Since the sum ofthe blending filters is always one, this is equivalent to the blendingassist torque τ_(ba) being equal to the input torsion bar torque τ_(s)multiplied by (S_(c))_(max), i.e., ##EQU3## The gain (S_(c))_(max)becomes part of the open loop transfer function. A filter is thendesigned for the open loop system (G_(p) ×(S_(c))_(max)) to achieveperformance and stability requirements. In one embodiment of theinvention, the maximum assist gain (S_(c))_(max) is 5.

One skilled in the art will appreciate from FIGS. 5A and 5B that if again of 5 (or 14 db) is added to the gain portion of the Bode plot, thesystem will have insufficient stability margins. Therefore a filter isadded to the open loop system to achieve desired performance andstability objectives. In accordance with a preferred embodiment, afilter of the form: ##EQU4## is used. The filter G_(f) is a lag-leadtype filter that is designed to provide a system with adequateperformance and stability margins at a maximum steering system gain of5. It can be seen from FIGS. 4A and 4B that for a gain of S_(c) =5, thesystem has approximately 10 db of gain margin, and 35 degrees of phasemargin. The open loop transfer function shown includes three quantitiesthat describe the behavior of the open loop steering system: (i) themaximum local assist curve gain (S_(c))_(max), (ii) the torque filterG_(f), and (iii) the measured transfer function G_(p).

Since the torque filter is designed to accommodate the maximum gain ofthe assist curve, the system will always be stable for assist curvegains below (S_(c))_(max). In actual operation of the steering system,the gain of the assist curve circuit S_(c) can change from low valuesall the way up to (S_(c))_(max). FIG. 6 illustrates the open looptransfer function of a steering system for gains of 1<S_(c) <5. The openloop transfer function G_(c1) as shown includes the measured steeringsystem transfer function G_(p), the torque filter designed for themaximum assist gain G_(f), and the effects of the blending filter.G_(ba) is defined as the transfer function from the torsion bar measuredtorque τ_(s) to the blended assist torque τ_(ba) : ##EQU5## The openloop transfer function shown in FIG. 6 are

    G.sub.c1 =G.sub.p G.sub.f G.sub.ba

In FIG. 6, the gain S_(c1) is set to 5 which is the same as(S_(c))_(max) in the preferred embodiment of the present invention. Thegain S_(c) is the local gain of the assist curve circuit for somenominal input torque and vehicle speed. The zero frequency or DC gain ofthe open loop transfer function G_(c1) is S_(c) and all of the transferfunctions cross over the zero db line (referred to as the crossoverfrequency) at 32 Hz. This indicates that all curves have the samebandwidth or time domain response characteristics yet all have differentDC or low frequency response due to the blending filter and the assistcurve circuit 54.

FIG. 7 illustrates the transfer function G_(f) G_(ba). The zerofrequency or DC gain of the transfer function is always S_(c) and yetthe high frequency gain is S_(c1). The use of the blending filters hasallowed the DC response characteristic of the compensation to bedifferent from the high frequency gain characteristics. Thischaracteristic of the blending filters allows the system to have anydesired feel via speed-pro for low frequency inputs, and yet be veryresponsive to quick steering inputs.

Referring to FIG. 6, it can be seen that if the gain at 32 Hz is reducedin the open loop transfer function, the bandwidth of the steering systemwill also be reduced. Because of the blending filters, the highfrequency gain can be reduced by setting S_(c1) to a lower value. Sincethe high frequency assist gain is a function of vehicle speed, thebandwidth of the steering system at high speed can be reduced ifnecessary by reducing S_(c1) as a function of speed.

In one embodiment of the present invention, the blending filter polew_(b) is chosen to be about 1 decade less than the crossover frequencyof 32 Hz, i.e., 3.2 Hz. FIG. 8 illustrates the frequency response forthe transfer function G_(ba). The gain S_(c) increases from 0.5 to 8 atincrements of 0.5. The high frequency assist gain S_(c1) is set equal to5. For the case shown, the maximum assist gain is higher than the highfrequency assist gain. The steering control system does not suffer fromeither performance nor stability problems when S_(c1) is greater thanS_(c) because there is no large gain or phase change at the zero dbcrossover frequency of 32 Hz. so the stability margins of the steeringsystem have not changed. The DC gain of G_(ba) is S_(c) and the highfrequency gain is S_(c1).

As long as the blending frequency is approximately one decade lower thanthe high gain crossover frequency (as designed for the case of maximumassist gain), the system will always be stable as long as the local gainis not much greater than the gain S_(c1). It is also possible toindependently lower the crossover frequency of the system at any vehiclespeed by lowering S_(c1) as the vehicle speed increases.

Those skilled in the art will appreciate that the low pass blendingfilters and the adaptive torque filter are realized in a digitalcomputer as digital filters using pole-zero mapping. Basically, thepoles P and zeros Z of the continuous system are mapped to the poles pand zeros z of the discrete digital filter via:

    p=exp(Pt), and z=exp(Zt),

where exp is the natural exponent and t is the sample rate. The samplerate of the digital filters, in accordance with one embodiment of thepresent invention, is approximately 300 micro-seconds. With pole zeromapping, the digital filter is then "forced" to have the same gain DC asthe continuous filter.

Those skilled in the art will appreciate that the blending and adaptivetorque filters will maintain a selectable system bandwidth independentof vehicle speed and assist curve gain changes.

From the above description of preferred embodiments of the invention,those skilled in the art will perceive improvements, changes andmodifications. Such improvements, changes and modifications within theskill of the art are intended to be covered by the appended claims.

Having fully described the invention, the following is claimed:
 1. Anapparatus for controlling a steering assist system, said steering assistsystem providing assist in response to a steering control signal, saidapparatus comprising:torque sensing means operatively connected to avehicle hand wheel for providing a torque signal indicative of appliedsteering torque; blending filter means connected to said torque sensingmeans for providing a blended filtered torque signal having a firstfunctional characteristic at torque frequencies less than a blendingfrequency and a second functional characteristic at torque frequenciesgreater than said blending frequency; steering assist means forproviding steering assist in response to a control signal; and controlmeans operatively connected to said blending filter means for providingsaid control signal to said steering assist means in response to saidblended filtered torque signal, said blending filtering means filteringsaid torque signal so as to maintain a selectable system bandwidthduring system operation.
 2. The apparatus of claim 1 wherein said firstfunctional characteristic of said blending filter is non-linear andwherein said second functional characteristic is linear.
 3. Theapparatus of claim 1 wherein said control means further includesadaptive filter means for filtering said blended filtered torque signalso as to maintain system stability at all vehicle speeds.
 4. Theapparatus of claim 1 wherein said blending filter means includes lowpass filter means for passing the torque signal with frequencies lessthan said blending frequency.
 5. An apparatus for controlling a steeringassist system, said steering assist system providing assist in responseto a steering control signal, said apparatus comprising:vehicle speedsensing means for sensing vehicle speed and for providing a speed signalhaving a value indicative of the sensed vehicle speed; torque sensingmeans operatively connected to a vehicle hand wheel for providing atorque signal indicative of applied steering torque; blending filtermeans connected to said torque sensing means and to said vehicle speedsensing means for providing a blended filtered torque signal having afirst functional characteristic at torque frequencies less than ablending frequency and a second functional characteristic at torquefrequencies greater than said blending frequency, said blending filtermeans further establishing said blending frequency at a valuefunctionally related to vehicle speed; steering assist means forproviding steering assist in response to a control signal; and controlmeans operatively connected to said blending filter means for providingsaid control signal to said steering assist means in response to saidblended filtered torque signal, said blending filtering means filteringsaid torque signal so as to maintain a selectable system bandwidthduring system operation.
 6. The apparatus of claim 5 wherein said firstfunctional characteristic of said blending filter is non-linear andwherein said second functional characteristic is linear.
 7. Theapparatus of claim 5 wherein said control means further includesadaptive filter means for filtering said blended filtered torque signalso as to maintain system stability at all vehicle speeds.
 8. Theapparatus of claim 5 wherein said blending filter means includes highpass filter means for passing the torque signal with frequencies greaterthan said blending frequency.
 9. The apparatus of claim 5 wherein saidblending filter means includes low pass filter means for passing thetorque signal with frequencies less than said blending frequency.
 10. Amethod for controlling a steering assist system that provides steeringassist in response to a steering control signal, said method comprisingthe steps of:measuring applied steering torque and providing a torquesignal indicative of the measured applied steering torque; filteringsaid torque signal so as to have a first functional characteristic attorque frequencies less than a blending frequency and a secondfunctional characteristic at torque frequencies greater than saidblending frequency so as to maintain a selectable system bandwidthduring system operation; providing steering assist in response to asteering control signal; and providing the control signal in response tothe filtered torque signal.
 11. The method of claim 10 wherein saidfiltering step includes filtering said torque signal so as to have afirst non-linear characteristic at torque frequencies less than theblending frequency and a linear characteristic at torque frequenciesgreater than said blending frequency.
 12. The method of claim 10 whereinsaid step of filtering includes further filtering said filtered torquesignal so as to maintain system stability at all vehicle speeds.
 13. Themethod of claim 10 further including the step of establishing saidblending frequency value one decade lower than the system's open loopzero db crossover frequency.
 14. An apparatus for controlling a steeringassist system, said steering assist system providing assist in responseto a steering control signal, said apparatus comprising:torque sensingmeans operatively connected to a vehicle hand wheel for providing atorque signal indicative of applied steering torque; blending filtermeans connected to said torque sensing means for providing a blendedfiltered torque signal having a first functional characteristic attorque frequencies less than a blending frequency and a secondfunctional characteristic at torque frequencies greater than saidblending frequency; steering assist means for providing steering assistin response to a control signal; and control means operatively connectedto said blending filter means for providing said control signal to saidsteering assist means in response to said blended filtered torquesignal, said blending filtering means filtering said torque signal so asto maintain a selectable system bandwidth during system operation;wherein said blending filter means includes means for establishing ablending frequency value one decade lower than the system's open loopzero db crossover frequency.
 15. An apparatus for controlling a steeringassist system, said steering assist system providing assist in responseto a steering control signal, said apparatus comprising:torque sensingmeans operatively connected to a vehicle hand wheel for providing atorque signal indicative of applied steering torque; blending filtermeans connected to said torque sensing means for providing a blendedfiltered torque signal having a first functional characteristic attorque frequencies less than a blending frequency and a secondfunctional characteristic at torque frequencies greater than saidblending frequency, and including high pass filter means for passing thetorque signal with frequencies greater than said blending frequency;steering assist means for providing steering assist in response to acontrol signal; and control means operatively connected to said blendingfilter means for providing said control signal to said steering assistmeans in response to said blended filtered torque signal, said blendingfiltering means filtering said torque signal so as to maintain aselectable system bandwidth during system operation.
 16. An apparatusfor controlling a steering assist system, said steering assist systemproviding assist in response to a steering control signal, saidapparatus comprising:vehicle speed sensing means for sensing vehiclespeed and for providing a speed signal having a value indicative of thesensed vehicle speed; torque sensing means operatively connected to avehicle hand wheel for providing a torque signal indicative of appliedsteering torque; blending filter means connected to said torque sensingmeans and to said vehicle speed sensing means for providing a blendedfiltered torque signal having a first functional characteristic attorque frequencies less than a blending frequency and a secondfunctional characteristic at torque frequencies greater than saidblending frequency, said blending filter means further establishing saidblending frequency at a value functionally related to vehicle speed andone decade lower than the system's open loop zero db crossoverfrequency; steering assist means for providing steering assist inresponse to a control signal; and control means operatively connected tosaid blending filter means for providing said control signal to saidsteering assist means in response to said blended filtered torquesignal, said blending filtering means filtering said torque signal so asto maintain a selectable system bandwidth during system operation;wherein said blending filter means further includes means forestablishing said blending frequency value.